Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce cyan, magenta, and yellow electrical signals. These signals can be transmitted to a thermal printer. To obtain a print, a black, cyan, magenta, or yellow dye-donor layer, for example, can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly, which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor sheet. The thermal print head can be heated up sequentially in response to the black, cyan, magenta, or yellow signals. The process can be repeated as needed to print all colors, and a laminate or protective layer, as desired. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal transfer works by transmitting heat through the donor from the backside to the dye-donor layer. When the dyes in the dye-donor layer are heated sufficiently, they sublime or diffuse, transferring to the adjacent dye-receiving layer of the receiver element. The density of the dye forming the image on the receiver can be affected by the amount of dye transferred, which in turn is affected by the amount of dye in the dye-donor layer, the heat the dye-donor layer attains, and the length of time for which the heat is maintained at any given spot on the donor layer.
At high printing speeds, considered to be 2.0 ms/line or less, the print head undergoes heat on/off cycles very rapidly. This generated heat must be driven through the dye donor support assemblage very rapidly to affect the dye transfer from the donor to the receiver. Each layer in the donor can act as an insulator, slowing down the heat transfer through the layers of the donor to the receiver. Because of the short heat application time, any reduction in heat transfer efficiency results in a lower effective temperature in the donor layer during printing, which can result in a lower transferred dye density. It is known to overcome the low print density associated with shorter line times by increasing the printhead voltage, increasing the dye density in the dye donor layer, or a combination thereof. Applying higher print head voltages can decrease the lifetime of the thermal print head, and requires a higher power supply, both of which increase cost. Increasing the dye density in the dye-donor layer increases costs, as well as increasing the chance of unwanted dye transfer, such as during storage of a dye-donor element.
Another problem exists with many of the dye-donor elements and receiver elements used in thermal dye transfer systems. During storage, the dyes can crystallize due to changes in temperature, humidity, or both. Crystallization of the dye can produce areas of non-uniformity in printing, resulting in dye-dropout from printed images.
Crystallization, light fade, and density of magenta dyes and dye combinations are known problems. Various dyes and dye combinations have been introduced in an attempt to produce a satisfactory printed color tone, while achieving sufficient print density at varying speeds, acceptable light fade, and an acceptable level of crystallinity. Examples of such dyes and dye combinations can be found in U.S. Pats. Nos. 4,839,336; 5,476,943; 5,532,202; 5,300,475; and Reissue 33,819. Similar dye structures can also be found in U.S. Pat. No. 6,866,706 B2.
There is still a need for magenta dye combinations that provide improved lightfastness, have improved keeping properties, are more efficient, or provide sufficient density when printing at various speeds, including high speeds of 2 ms or less per line.